Microsystem-Enabled Photovoltaic (MEPV) cells allow solar PV systems to take advantage of scaling benefits that
occur as solar cells are reduced in size. We have developed MEPV cells that are 5 to 20 microns thick and down to 250
microns across. We have developed and demonstrated crystalline silicon (c-Si) cells with solar conversion efficiencies of
14.9%, and gallium arsenide (GaAs) cells with a conversion efficiency of 11.36%. In pursuing this work, we have
identified over twenty scaling benefits that reduce PV system cost, improve performance, or allow new functionality.
To create these cells, we have combined microfabrication techniques from various microsystem technologies. We have
focused our development efforts on creating a process flow that uses standard equipment and standard wafer thicknesses,
allows all high-temperature processing to be performed prior to release, and allows the remaining post-release wafer to
be reprocessed and reused. The c-Si cell junctions are created using a backside point-contact PV cell process. The GaAs
cells have an epitaxially grown junction. Despite the horizontal junction, these cells also are backside contacted. We
provide recent developments and details for all steps of the process including junction creation, surface passivation,
metallization, and release.
In the fabrication of MEMS devices, what has come to be known as "release stiction" occurs when the device is removed from the liquid phase into the ambient air. One widely used method for dealing with stiction is to deposit a hydrophobic coating on the surface of the device before it is removed from the liquid phase. This method can produce coatings with inconsistent morphology and device yield. This is to be compared with a new coating deposition scheme developed at Sandia National Labs, termed VSAMS (vapor-deposited self-assembled monolayers) that employs supercritical CO2 drying and chemical vapor deposition to address many of the concerns associated with release stiction. VSAMS is attractive due to its process benefits, which include increased throughput, reduced waste, and most importantly, it can be easily scaled to full wafer production. It is also attractive because films produced by this method are uniform and very hydrophobic. The deposition step makes use of a class of compound that is particularly suited for vapor phase reactions, amino-functionalized silanes. The yield of microengine test devices coated with films made from amino-functionalized silanes was examined over an extended period. Their function was determined before and after the application of VSAMS. The advantage of using amino-functionalized silane precursors for VSAMS is related to the strength of the bond between the film and the polysilicon surface as evidenced by the fact that films made with these precursors are stable across the entire humidity scale.
Supercritical CO2 drying has been shown to be an effective method for drying complex MEMS structures with little or no stiction. This process typically involves transferring released parts from ultrapure water into a solvent, such as methanol, that is miscible with liquid CO2, and subsequently displacing the methanol with liquid CO2. During this process sequence, transport of methanol into and out of the tortuous pathways of the MEMS device is dominated by diffusion. The non-steady state diffusion equation (Fick’s second law) with length scales relevant to MEMS structures can be applied to understand the speed at which diffusion occurs. This analysis indicates that diffusion of methanol into the pathways of a MEMS device occurs very rapidly, typically on the order of minutes. Experimental data are consistent with the rapid diffusion hypothesis.
Stiction induced by capillary forces during the post-release drying step of MEMS fabrication can substantially limit the functional yield of complex devices. Supercritical CO2 drying provides a method to remove liquid from the device surface without creating a liquid/vapor interface, thereby mitigating stiction. We show that a continuous stirred-tank reactor (CSTR) model can be applied as a method to estimate the volume of liquid CO2 required to effectively displace the post release solvent. The CSTR model predicts that about 8 volume exchanges is sufficient to effectively displace the methanol to a concentration below the saturation point. Experimental data indicate that about 10 exchanges are adequate for repeatable drying of complex devices, which is in reasonable agreement to the model prediction. In addition to drying devices without inducing stiction, the process must be inherently non-contaminating. Data indicate that the majority of contaminants deposited during the drying process can be attributed to contaminants originating in the post-release solvent, rather than the supercritical CO2 process.
Selectively deposited tungsten films on MEMS surfaces that are subject to friction and wear can substantially reduce wear-related failures. Because deposition of the tungsten film is highly selective to silicon, a pristine surface is required to obtain high quality, contiguous films. Vapor phase HF was used to remove the thin chemical oxide that resides on the surface following traditional liquid phase dissolution of sacrificial oxide films and supercritical CO2 drying of MEMS devices. The use of vapor phase HF after mechanical parts have been released, rather than liquid processes, mitigates potential device damage and surface tension-induced stiction that may occur during liquid phase processing. Tungsten film thickness and morphology were identical to films that were obtained through the use of liquid phase pre-cleaning processes.
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